Radiographic imaging, in its simplest expression, is an X-ray beam traversing an object and a detector relating the overall attenuation per ray. The attenuation is derived from a comparison of the same ray with and without the presence of the object. From this conceptual definition, several steps are required to properly construct an image. For instance, the finite size of the X-ray generator, the nature and shape of the filter blocking the very low-energy X-ray from the generator, the details of the geometry and characteristics of the detector, and the capacity of the acquisition system are all elements that affect how the actual reconstruction is performed.
Typical CT systems include an X-ray source and stationary photon-counting detectors (PCDs). The PCDs are configured to acquire spectral data, which is then decomposed into line integrals of the basis material of the imaged object. The basis images of the object can be reconstructed from the line integrals and thus can provide material information of the imaged object. However, the PCDs experience high flux levels during object scanning and may not be able to function correctly. Thus, in order to obtain reliable spectral information of the imaged object, the problem of the high flux in spectral CT apparatus needs to be addressed.